Electrical keyboards for manual input to data processing system have increasingly adopted capacitive circuitry in recent years since capacitive keyswitches, when properly designed, may provide reliable operation at low cost. In electrical keyboards, however, it is extremely desirable to provide an overtravel mechanism, which means that the key plunger may be depressed beyond the minimum amount which is needed to activate the data processing equipment.
A number of advantages are obtained from such an overtravel mechanism. First, the use of overtravel requires that the plunger will be depressed beyond the minimum amount that is required to insure position of the switching action. Next, the use of overtravel tends to result in a reduction of the bounce that otherwise might occur since the overtravel mechanism acts as a shock absorber when the plunger is depressed. Furthermore, with overtravel, the plunger will be depressed at least a minimum period which helps to discriminate against noise or teasing of the switch.
A simple overtravel mechanism which is formed by a tab cut out of an electrical contact arm that is mounted at an acute angle in a switch is found in the Golbeck et al. U.S. Pat. No. 3,916,131, issued Oct. 28, 1975. This same type of inexpensive overtravel mechanism is found in a capacitive switch in the Cencel U.S. Pat. No. 4,090,229 wherein it is combined with a pair of elongated leaf spring hinges which are formed by slots in the metal moveable capacitor plate.
The cut-out tab overtravel mechanism of the Golbeck et al. and Cencel et al. patents is a cost-effective means of obtaining overtravel in a capacitive switch. However, since the tab is separated from the remainder of the capacitor plate in the Cencel device, an irregular variation in the capacitance of the keyswitch can occur as the tab is flexed. In addition, in the Cencel et al. patent, the elongated springs that are formed by slots in the capacitive plate can cause another undesired variation in the capacitance as the plate is depressed. In a capacitive keyswitch, it is desirable, of course, to minimize any variations that can occur due to slots or openings in the moveable capacitive plate itself. In addition, it has been found that if the moveable capacitive plates are constructed as shown in the Cencel et al. patent, they tend to bow upon full depression due to the particular hinging arrangement that is employed, thereby causing further unwanted variations of the capacitance between the plates of the keyswitch.
The present invention is directed to an overtravel mechanism which allows the moveable capacitive plate to be fully depressed with virtually no bowing and which, in addition, eliminates the need for cutting the overtravel tab out of the moveable plate.